Controlled collapse of depressed index optical fiber preforms

ABSTRACT

The doped silica core region of a core rod for an optical fiber preform is protected against unwanted fluorine doping during fluorine doping of the outer silica layer by selectively consolidating the core region prior to fluorine doping. Due to dopants in the core region, the soot in the core region consolidates before the soot in the outer undoped region. This inherent property allows the entire core rod to be heated prior to fluorine doping resulting in selective partial consolidation and preventing fluorine doping of the doped center core region. The process time required may be reduced by using incremental fluorine doping. In the incremental doping process the doping step is separated into a deposit step, where “excess” fluorine is deposited on the silica particles, and a drive-in step where atomic fluorine is distributed into the silica particles. The drive-in step is conveniently combined with the sintering or consolidation step to further enhance the efficiency of the doping process.

FIELD OF THE INVENTION

[0001] This invention relates to methods for making depressed clad indexoptical fibers and is directed more specifically to techniques forpreparing preforms prior to drawing optical fibers.

BACKGROUND OF THE INVENTION

[0002] Depressed clad optical fibers were developed in the early 1980'sas an alternative to fibers with doped cores and less heavily doped, orundoped cladding. See, e.g., U.S. Pat. No. 4,439,007. Depressed claddingallows the use of fiber cores with relatively low doping. These coresprovide low optical loss.

[0003] Applications have been developed for both single mode andmultimode depressed clad fibers, and a variety of processes for themanufacture of depressed clad fibers were also developed. See e.g. U.S.Pat. No. 4,691,990, the disclosure of which is incorporated herein byreference. Complex refractive index profiles in optical fibers often usecore regions of down doped silica.

[0004] Optical fibers with down doped regions have been found especiallyuseful for lightwave systems in which control of non-linear effects isimportant. For example, in four-wave mixing of optical frequencies inthe 1.5-1.6 mm wavelength region where DWDM networks operate, a lowslope, low dispersion fiber is required. Recent advances in opticalfiber technology have extended the DWDM range of operation to providevery high capacity transmission over a single fiber. Among theseadvances are fibers with nonzero-dispersion which are specificallydesigned to overcome pulse broadening and signal mixing in high poweroptically amplified DWDM systems over long distances. Current typicaloptically amplified DWDM systems operate in the 1530 to 1565 nmwavelength range, or the third window in the fiber spectrum. Emergingsystems will use the fourth window (1565 to 1620 nm) to increase networkcapacity and optimize performance.

[0005] One of the important parameters in fiber designed for ultrahigh-speed networks is dispersion slope. Managing dispersion in thefiber itself reduces the need for high cost dispersion compensationcomponents when used in high capacity WDM amplified systems.

[0006] Optical fibers for these and other advanced designs often requirefiber

[0007] cores with a down doped trench just outside the core region.These fibers have a modified W shaped index profile that has been foundto be efficient for single mode guiding with low loss, and can bedesigned to have the dispersion characteristics mentioned above.

[0008] One technique for making depressed index regions in optical fiberpreforms is to dope the region with fluorine or boron. In the case of aW shaped index fiber, the region of the core trench has a refractiveindex less than silica. In a preferred form, the center of the core isdoped with, e.g. germania, to increase the index. In these structures,the Δn is relatively large,

[0009] e.g. 0.005-0.010, between the center of the core and trench. TheΔn between the trench and a silica cladding layer may be less than halfthat Δn value.

[0010] In the manufacture of preforms for these fibers, one approach isto down dope the outer layer of a porous core rod (the shell) by“soaking” the core rod in a fluorine containing gas atmosphere with thecore rod still in the porous state, i.e. prior to consolidation. Theporosity of the core rod at this stage in the process allows thefluorine gas to easily permeate the germania doped silica body. Theporosity is typically in the range of 50-90%, measured as volume ofsolids to volume of voids. The conventional practice is to diffusefluorine into the silica body using an equilibrium doping process. Inthis process, the silica body is heated to a temperature of rapiddiffusion, in the presence of a low partial pressure of fluorine, i.e. apartial pressure sufficient to supply a continuous flux of fluorine tomaintain the equilibrium diffusion. However, recognizing that the centerportion of the core requires a higher index and cannot therefore be downdoped, it has been difficult to confine the doping process to the corerod “shell” in a controllable and predictable way. Fiber preforms withdown doped core regions and a center region doped with a conventionalindex increasing dopant such as germania allow some latitude in theselective doping of the shell. One approach has been to dope the centerof the core with an excess of germania, and down dope the entire corerod with fluorine. This produces a depressed index profile in the shell,but at added cost and with added optical loss in the core.

[0011] Another approach to selective down doping of the core rod shellhas been described by Kanamori et al., in U.S. Pat. No. 5,055,121. Thisapproach uses a solid core rod onto which a soot layer is formed. Theporous soot layer can then be doped with SiF₄, which permeates the sootlayer rapidly but diffuses slowly into the solid glass rod. In this wayfluorine doping is confined to the shell region outside the core. Thisapproach may be used for both down doping a shell region of a preform aswell as for down doping a cladding of a preform. However, this approachis complex and expensive, requiring separate processing for the corerod, the shell, and the overcladding.

SUMMARY OF THE INVENTION

[0012] We have developed a fluorine doping process for optical fiberpreforms .with a “W” index profile that allows controlled doping of aporous silica core rod with fluorine but inhibits fluorine doping of thegermania doped core. A key step in this process is a preliminary partialconsolidation step wherein the germania core region is selectivelyconsolidated prior to fluorine doping. The remainder of the core rod,still in a porous state, is doped with fluorine and then consolidated.The preliminary selective consolidation step protects the germania dopedcenter region of the core rod from fluorine penetration. In a preferredembodiment of the invention, incremental fluorine doping is used. SeeU.S. patent application Ser. No. 09/755,914 filed Jan. 5, 2001. Thisfluorine doping process is relatively rapid, which further aids inpreventing substantial fluorine doping of the center core region.

BRIEF DESCRIPTION OF THE DRAWING

[0013]FIG. 1A is a refractive index profile for a W shaped index fiberthat can be made using the process of the invention;

[0014]FIG. 1B is a refractive index profile for a soot body, identicalto that used for FIG. 1A, but without pre-sintering before fluorinetreatment.

[0015]FIG. 2 is a representation of a porous core rod in a fluorinedoping furnace;

[0016]FIGS. 3 and 4 are schematic views of a section through a soot bodybefore (FIG. 3) and after (FIG. 4) partial consolidation of the sootbody according to the invention;

[0017]FIG. 5 is a plot of doping time vs. refractive index change forequilibrium doping processes;

[0018]FIG. 6 is a schematic view of a section through a silica particletreated by the incremental doping process of the invention showing thedopant distribution after the deposit step;

[0019]FIG. 7 is a representative plot of dopant concentration vs.distance for the particle of FIG. 6;

[0020]FIG. 8 is a schematic view of a section the particle of FIG. 6showing the impurity distribution after the drive-in step;

[0021]FIG. 9 is a representative plot of dopant concentration vs.distance through the particle of FIG. 6; and

[0022]FIG. 10 is a schematic representation of an optical fiber drawingapparatus.

DETAILED DESCRIPTION

[0023]FIG. 1A shows a typical refractive index profile for a W profileoptical fiber of the kind to which the invention, in a principleembodiment, is directed. The center core region extends between radius−10 and +10. The depressed clad region extends from −10 to +35 and from+10 to +35. The silica clad region extends from −35 and +35 to thesurface of the preform. The problem that is addressed by the inventionis to dope the depressed core region of the initial core rod (the shell)without doping the center germania doped core region. The center coreregion, as seen, transitions between the fluorine doped depressed regionand the up-doped center of the core. The center of the core is dopedtypically with germania. If fluorine penetrates to the center of thecore during doping then the Δn will be lowered and the fiber propertiescompromised. The refractive index profile for a preform doped withfluorine in the manner of the invention, i.e. preserving the germaniadoped index change in the core, is illustrated by FIG. 1A. A refractiveindex profile for a preform doped by a prior art method, where thecenter core region is undesirably down-doped, is shown in FIG. 1B.

[0024] A process sequence for avoiding fluorine doping of the center ofthe core will be described in the context of an incremental fluorinedoping process. It should be understood that the incremental dopingprocess is a preferred embodiment of the invention, and that theexpedient used to prevent fluorine doping of the center of the core canbe practiced with other forms of fluorine doping, e.g., equilibriumdoping. The feature that allows the objective of the invention to berealized is partial consolidation of the core prior to inhibit fluorinedoping of the germania doped region. This feature can easily beimplemented using any normal fluorine doping technique.

[0025] Likewise, the invention is described here in the context of acore rod and silica tube process. Other index profiles can be obtainedusing the process of the invention. For example, an up-doped core and adown-doped cladding may be produced using, e.g. a VAD process. Thecladding may be down-doped minimizing the effect on the germania coreregion using the technique of the invention. Partial consolidation ofthe core region, practiced according to the invention, generally has anup-doped center core region as a prerequisite. The center core regionmay be doped with any dopant that produces a soot that consolidates at alower temperature than the soot of the surrounding region. (IReferenceto soot in this context is intended to include porous glass materialdeposited by any known technique). In principle, any degree of doping ofthe center of the core will produce this condition, but it is preferredfor the purpose of the invention, that the center of the core have adopant concentration, in terms of germania, of at least 2 wt. %. In moregeneral terms, it is prescribed that the dopant in the center of thecore be sufficient to produce an index increase of at least 0.002 overthat of pure silica.

[0026] The preferred process of the invention, i.e. incremental doping,will be described in conjunction with FIGS. 2-10. With reference to FIG.2, a porous core rod 11, is shown prior to consolidation into a rodpreform suitable for optical fiber manufacture. For consistency innomenclature, and clear understanding of the invention, the term preformwill refer to the final structure after consolidation or after assemblyof the rod and tube (frequently referred to as jacketing), and collapseof the tube around the rod. The preform stage is the final fabricationstage of the glass prior to drawing the optical fiber. Where used, theterm cladding tube refers to the glass tube into which the core glassrod is inserted to make a preform. In the context of core rods made byVAD or other equivalent process, the rod may be referred to as porous,i.e. prior to consolidation.

[0027] For convenience in describing the incremental doping process ofthe invention the example used in this description is a core rod made bya VAD process. The basic VAD process is well known and requires nodetailed D1 o explanation here. For reference, see Optical Fibers:Materials and Fabrication by T. Izawa and S. Sudo, KTK Scientificpublishers, Tokyo, 1978. It will be understood that the inventionapplies equally to the manufacture of optical fibers using preforms madeby any suitable technique that results in a porous body with an up-dopedcenter core region and a surrounding porous layer that is to be fluorinedoped. Useful techniques may include VAD, CVD, MCVD, etc.

[0028] A porous VAD rod, suitable for making fiber with the desiredprofile, typically has a soot diameter of 100 to 400 mm. The VAD rod,after consolidation and shrinkage, will have dimensions represented byFIG. 1, i.e. a diameter in general of 50 to 200 mm and, as illustrated,approximately 70 mm. The core rod has a center core region dopedtypically with GeO₂ to a Δn˜0.003 or greater. While in the soot state,and prior to fluorine doping, the rod is heated to a temperature in therange 1150° C. to 1400° C. for a period of 5-240 minutes to partiallyconsolidate the germania doped portion of the rod. The exact processconditions for partial consolidation will depend on several variablesincluding the soot body dimensions, concentration of dopant in the core,and particle size. Complex index profiles may involve multiple dopedlayers, with a center region doped higher than surrounding regions. Herethe suitable partial consolidation conditions will be determined by theconcentration of dopant in the lowest Δn layer, as well as the coreregion. The partial consolidation conditions can easily be determinedempirically by those skilled in the art. The process of the inventionwill be characterized by heating the prepared soot rod to a temperatureof at least 1100° C. prior to fluorine doping, e.g. in the dopingfurnace but in an atmosphere essentially devoid of fluorine.

[0029] If the incremental doping process is used, according to apreferred embodiment of the invention, then the process will becharacterized by a preliminary heating step in the absence of fluorineto partially consolidate the core part of the soot body, a heating stepin a fluorine atmosphere, and a final heating step without fluorine forthe drive-in/consolidation step characteristic of the incremental dopingprocess. The latter will be described in more detail below.

[0030] The soot body is shown prior to partial consolidation in FIG. 3.The soot body is designated 13, with a germania doped center core region14, and undoped (or lower doped) silica surrounding region 15. The sootbody is shown after partial consolidation in FIG. 4, where the germaniadoped center core region 14 has been densified, lowering the porosityand surface area, leaving a surrounding layer 15 of undoped silica sootthat remains unconsolidated. The surrounding soot region 15 remainsporous and susceptible to rapid penetration by fluorine duringsubsequent fluorine doping.

[0031] Following the partial consolidation step, just described, thesoot body is treated in a fluorine gas atmosphere to provide thefluorine dopant for the outer (porous shell) region of the partiallyconsolidated body. This step is illustrated in FIG. 2 where the porousbody 11 is shown schematically being treated in furnace 12. Asmentioned, the fluorine doping step will be described as an incrementaldoping process. For comparison, the description begins with a briefexplanation of the standard equilibrium doping process. The usualfluorine source is SiF₄with a carrier gas such as nitrogen, argon orhelium. The furnace atmosphere is heated to a temperature in the range500-1200° C. for this step. Molecular SiF₄ permeates into the outside ofthe porous body and rapidly penetrates the entire rod. At the same time,diffusion of atomic fluorine begins at the particle level. This effectis termed diffusion to define doping of the individual silica particleswith elemental fluorine. The diffusion front proceeds from the surfaceof the particle, which is exposed to the fluorine atmosphere, to thecenter of the particle. It is recognized that the diffusion in theequilibrium doping process is inherently slow. The Δn is proportional tothe SiF₄ partial pressure to the quarter power:

Δn˜p ^(1/4)  (1)

[0032] The equilibrium partial pressures of SiF₄ corresponding to dopinglevels of Δn=0.001-0.003 are 1×10⁻⁴-8.0×10^(×3), respectively. Thedoping of large porous bodies at intermediate and high SiF₄ partialpressures is generally limited by mass transport in and into the porousbody. However, in the low partial pressure regime used in equilibriumdoping, the rate of SiF₄ introduction into the reaction vessel becomesthe rate limiting step. Recently, the refractive index variation wasfound to be linearly related to the weight of SiF₄ incorporated. Thiscan be used to estimate the dependence of the doping time scale on theSiF₄ partial pressure under this limiting condition. The doping timescale, t_(D), can be defined by the ratio of the desired Δn times thebody mass divided by the SiF₄ partial pressure, SiF₄ mass, and the totalflow, corrected by a doping efficiency, η, which is the fraction of SiF₄incorporated in the sample to the total amount supplied. In terms of thetotal molar flow into the reactor, F, sample weight, W, and refractiveindex variation Δn, the doping time scale, t_(D), is given by:

t _(D)α(ΔnW)/PFM)(1/η)˜KΔn/Δn ⁴(1/η)˜KΔn ⁻³(1/η);K˜W/(FM)  (2)

[0033] where M and P are the molecular weight of SiF₄ and the partialpressure, respectively. The second step in the development of equation 2uses the relationship from equation 1 (Δn˜p^(1/4)) Equation 2 shows thatfor a given sample size and SiF₄ flow rate, the doping time scale t_(D)is proportional to Δn⁻³. This is illustrated in FIG. 5. This exampleassumes 10 kg sample and 10 liters/min total volumetric flow. For thesame doping efficiency, an order of magnitude decrease in Δn results inan increase in doping time by three orders of magnitude. Doping times of5 and 130 hours are expected for Δn values of 0.003 and 0.001,respectively at 100% efficiency. Realistically, these doping times arepredicted to be an order of magnitude longer due to lower dopingefficiencies. In practical terms, increasing the flow rate may notdecrease doping times proportionally since this would result in anefficiency penalty due to the decrease in retention time of the gasdopant. Decreasing sample size may also reduce efficiency and, moreimportantly, compromises throughput. At the higher Δn values of FIG. 5,the predicted doping times due to introduction rates are very short.Thus the doping process in this regime is expected to be limited bydiffusion in the porous body, further decreasing doping efficiencies.However, at the low doping levels, Δn=0.001-0.003, the doping processclearly becomes very time consuming due to the low rate of SiF₄introduction into the reaction vessel.

[0034] To overcome the slow processing rates characteristic ofequilibrium doping processes a fundamentally different doping approachis used. A high SiF₄ partial pressure, well above the equilibriumpartial pressure, is used to deposit excess fluorine as a surface layeron the particles of the porous body. In some cases, this deposition stepmay be carried out at a temperature below the effective diffusiontemperature for practical distribution of fluorine throughout the silicaparticles, i.e. 1000° C. or below. A range of 800-1000° C. isrecommended although a wider range of 500-1100° C. may be found usefulin some cases. The fluorine deposits on the particle surface mainly bymolecular absorption, which is essentially instantaneous compared withthe time required for fluorine to penetrate substantially into theparticle by atomic diffusion. Thus the total amount of fluorine requiredfor the index modification of the preform is deposited as a concentratedsurface layer in a relatively short doping step. This deposition ofexcess fluorine on the surface of the particles of the porous body isthe first stage of the incremental doping process. Thereafter, theporous body is removed from the fluorine doping atmosphere (or decreasedto equilibrium) and is exposed to a high temperature drive-in step todistribute the excess fluorine, as deposited, uniformly throughout theporous silica body. This drive-in step, the second stage of theincremental doping process, may be combined with the consolidation stepof the process for further savings in process time.

[0035] The incremental doping process of the invention is illustrated inFIGS. 6-9. FIG. 6 shows an individual silica particle 21 of a poroussilica body being treated in a fluorine atmosphere. The deposited region24 on the outside of the particle 21 is a region containing a highconcentration of fluorine adsorbed on the surface of the particle. Theconcentration of fluorine in this surface layer is approximately inequilibrium with the atmosphere in furnace 22. The partial pressure offluorine can be adjusted as desired but is preferably at least 5 timesthe equilibrium partial pressure corresponding to the final doping levelfor the entire preform. This high partial pressure will depend on the Δndesired but will typically be greater than 1%. Using SiF₄ as thefluorine source, the deposition atmosphere in general will contain SiF₄in the range of 1-100%.

[0036] The temperature of the deposition may vary widely, and in anideal process depends in part on the porosity of the silica body. Forvery porous structures the SiF₄ permeation of the porous body mayproceed rapidly, and temperatures up to 1200° C. may be used. Forexample, VAD soots characteristically have porosity levels of 80% orgreater. VAD core rods 15 cm. in diameter can be uniformly doped to anincremental amount, Δn˜0.002, in approximately 10 minutes at 1000° C.For less porous structures, the permeation process is typically longer,and premature consolidation should be avoided during this step. At thelow end of the temperature range, e.g. 500° C., the deposition processis slower. This end of the range is useful for producing small Δnpreforms with a relatively high degree of control, i.e. the dopingconcentration level varies more slowly and controllably with time. Thistemperature dependence feature adds another dimension of control to theprocess. The final doping level of the preform can be controlled byadjusting the deposition temperature. In the equilibrium doping process,the doping level is controlled mainly by partial pressure of fluorine.The deposition step proceeds to completion in a period typically in therange of 10-240 minutes.

[0037] The preferred temperature range for the deposition step, to avoidany possibility of premature consolidation, is less than 1050° C. Forthe Δn values of most interest for doping core regions, i.e. relativelysmall Δn values, deposition temperatures of 1000° C. or less allow for adesired level of control.

[0038] The concentration of fluorine in the particle 21 of FIG. 6 isshown 20 schematically in FIG. 7 with axis x representing the diameterof the particle. The exterior region of the particle has a very highconcentration of fluorine, i.e. well above the target concentrationC_(T). In the preferred case, the concentration of fluorine at theparticle surface C_(I) will be at least ten times C_(T).

[0039] The drive-in or diffusion step is illustrated by FIGS. 8 and 9.The silica particle 21 in FIG. 8 is shown after drive-in with a uniformdistribution of fluorine throughout the particle. The fluorineconcentration profile for particle 21 is shown in FIG. 9, with uniformconcentration C_(T).

[0040] The drive-in step is preferably combined with the sintering stepand is conducted at temperatures in the range 1300-1600° C.Alternatively, the porous silica body may be treated at a temperature of1200-1400° C. for drive-in, then sintered. The process time required forthe drive-in/sintering step will vary depending on the size and geometryof the preform and the temperature used, but will typically be in therange 30-500 minutes. The overall process time, for both deposit anddrive-in/consolidation, may be less than an hour. This represents adramatic reduction in process time as compared with prior methods ofpreparing fluorine doped preforms.

[0041] The sintered perform is then used for drawing optical fiber inthe conventional way. FIG. 10 shows an optical fiber drawing apparatuswith preform 31, and susceptor 32 representing the furnace (not shown)used to soften the glass preform and initiate fiber draw. The drawnfiber is shown at 33. The nascent fiber surface is then passed through acoating cup, indicated generally at 34, which has chamber 35 containinga coating prepolymer 36. The liquid coated fiber from the coatingchamber exits through die 41. The combination of die 41 and the fluiddynamics of the prepolymer, controls the coating thickness. Theprepolymer coated fiber 44 is then exposed to UV lamps 45 to cure theprepolymer and complete the coating process. Other curing radiation maybe used where appropriate. The fiber, with the coating cured, is thentaken up by take-up reel 47. The take-up reel controls the draw speed ofthe fiber. Draw speeds in the range typically of 1-20 m/sec. can beused. It is important that the fiber be centered within the coating cup,and particularly within the exit die 41, to maintain concentricity ofthe fiber and coating. A commercial apparatus typically has pulleys thatcontrol the alignment of the fiber. Hydrodynamic pressure in the dieitself aids in centering the fiber. A stepper motor, controlled by amicro-step indexer (not shown), controls the take-up reel.

[0042] Coating materials for optical fibers are typically urethanes,acrylates, or urethane-acrylates, with a UV photoinitiator added. Theapparatus of FIG. 10 is shown with a single coating cup, but dualcoating apparatus with dual coating cups are commonly used. In dualcoated fibers, typical primary or inner coating materials are soft, lowmodulus materials such as silicone, hot melt wax, or any of a number ofpolymer materials having a relatively low modulus. The usual materialsfor the second or outer coating are high modulus polymers, typicallyurethanes or acrylics. In commercial practice both materials may be lowand high modulus acrylates. The coating thickness typically ranges from150-300μm in diameter, with approximately 240 μm standard.

[0043] The following examples are given to illustrate the invention.

EXAMPLE 1

[0044] A porous silica core rod, 150 mm in diameter, with a core centerdoped with germania to Δn˜0.005 is heated to 1100° C., dehydrated withchlorine, and purged with He. The core rod is then cooled to 1000° C.,and treated for 30 minutes in 10% SiF₄+He, to deposit SiF₄ on theparticles of the porous body. The porous body is then heated in heliumto 1500° C. and sintered for 1 hour to consolidate the entire body.

[0045] In this sample the clad has the desired index of approximately−0.002 Δn but the core has also been fluorine doped lowering therefractive index to +0.002. The refractive index profile for this caseis shown in FIG. 1B. This result is undesirable since the effect of thefluorine is to compensate or erase the effect of the germania dopant.Efforts to counteract the compensation effect by increasing the initialgermania doping level are only partially successful since the increasedoverall doping of the core increases the light absorption (loss) in thecore.

EXAMPLE 2

[0046] A porous VAD core rod with a core center doped with germania toΔn˜0.005 is heated in stages in He to 1100° C. in 7 hours, thendehydrated at that temperature with He/10% chlorine. The body is thenpartially consolidated by heating at 1275° C. for 2 hours. Thistreatment selectively consolidates the germania doped core but leavesthe remainder of the rod in a porous state. After cooling to 1000° C.,it is treated in He/0% SiF₄ for 30 minutes to deposit fluorine on thesurface of the particles. The deposited porous body is then heated to1550° C. for drive-in/consolidation. The finished rod has a Δn ofapproximately 0.005 in the core region, which is preserved from the Δnvalue prior to fluorine doping, and a Δn⁻ in the outer fluorine dopedregion (shell) of the core rod of approximately 0.002. The refractiveindex profile for this result is shown in FIG. 1A. As will beappreciated by those skilled in the art, this Δn⁻ was obtained using aSiF₄ treatment of 30 minutes, which compares favorably with theconventional equilibrium doping process in which treatment at theequilibrium partial pressure of SiF₄ of 2×10⁻³ atmospheres requires 20hours (assuming 100% efficiency, not typical in a production process).

[0047] In the foregoing description, the source of fluorine is SiF₄. Asevident to those skilled in the art, other sources of fluorine may beused. For example, SiHF₃, SiH₂F₂ , SF₆, CF₄, may also be suitable. Thesewill result in index depression equivalent to that produced by SiF₄.However, instead of contending with SiF₄ as low as 10⁻⁹ atmospheres, lowtemperature F⁻ doping can be regulated to provide the desired indexdepression at SiF₄ partial pressures above 1% and typically 10% or more.As indicated earlier, the 15 fluorine deposits on the individualparticles as primarily a molecular species. The drive-in mechanismprimarily involves diffusion of atomic fluorine. To amplify thisimportant distinction, the deposition step is described as involvingdeposition of fluorine on the particle surface, and the drive-in asdiffusion of fluorine.

[0048] The examples given above use germania as the core dopant but, asmentioned earlier, other dopant materials may be used to increase therefractive index in the core. These are referred to here categoricallyas up-dopants or materials that produce an up-doped region in the centerof the core. Likewise, other down dopants (other than fluorine) may befound useful for the outer shell layer of the core. These materials arecategorized generically as producing a “down-doped” region. Core rods tobe treated according to the invention will have, in general, an up-dopedcenter core region and a lower doped surrounding shell region. It isintended that the “lower” doped surrounding region includes the casewhere the surrounding region is undoped.

[0049] In concluding the detailed description, it should be noted thatit will be obvious to those skilled in the art that many variations andmodifications may be made to the preferred embodiment withoutsubstantial departure from the principles of the present invention. Allsuch variations, modifications and equivalents are intended to beincluded herein as being within the scope of the present invention, asset forth in the claims.

1. Process for the manufacture of doped silica bodies comprising: (a)preparing a porous body of silica particles, the porous body having afirst porous region that is up-doped and a second porous shell regioncomprising a lower doped or undoped portion, (b) heating the porous bodyto a temperature of at least 1100° C. in the absence of fluorine for aperiod sufficient to selectively consolidate the first porous region,(c) heating the porous body in a fluorine atmosphere to dope the silicaparticles in the second porous region, and (d) heating the porous silicabody at a temperature greater than 1300° C. to consolidate the poroussilica body.
 2. Process for the manufacture of optical fiberscomprising: (a) preparing an optical fiber preform, (b) heating thepreform to the softening temperature, and (c) drawing an optical fiberfrom the preform the invention characterized in that the optical fiberpreform is produced by: (i) preparing a porous body of silica particles,the porous body having a first porous region that is up-doped and asecond porous shell region comprising a lower doped or undoped portion,(ii) heating the porous body to a temperature of at least 1150° C. inthe absence of fluorine for a period sufficient to selectivelyconsolidate the first porous region, (iii) heating the porous body in afluorine atmosphere to dope the silica particles in the second porousshell region, and (iv) heating the porous silica body at a temperaturegreater than 1300° C. to consolidate the porous silica body.
 3. Processfor the manufacture of optical fibers comprising: (a) preparing anoptical fiber preform, (b) heating the preform to the softeningtemperature, and (c) drawing an optical fiber from the preform theinvention characterized in that the optical fiber preform is producedby: (i) preparing a porous silica core rod of silica particles, the corerod having an inner up-doped region surrounded by an outer shell region,said outer shell region comprising a lower doped region, (ii) heatingthe porous silica core rod to a temperature of at least 1100° C. in theabsence of fluorine for a period sufficient to selectively consolidatethe inner up-doped region, (iii) cooling the porous silica core rod (iv)introducing a fluorine-containing atmosphere with a first fluorineconcentration to deposit fluorine on the silica particles in the lowerdoped region, (v) reducing the fluorine concentration, and (vi) heatingthe porous silica body at a temperature greater than 1300° C., toconsolidate the porous silica core rod.
 4. The process of claim 3wherein the fluorine atmosphere comprises SiF₄.
 5. The process of claim4 wherein the fluorine atmosphere is greater than 10% SiF₄.
 6. Theprocess of claim 3 wherein the outer shell region is undoped silica. 7.The process of claim 3 wherein the inner core region is doped withgermania.
 8. The process of claim 7 wherein the inner core region isdoped to a Δn in the range 0.001-0.058.
 9. The process of claim 3wherein the temperature used in step (iii) is in the range 500-1100° C.